Sour gas comprising H2S can originate from various sources. For example, numerous natural gas wells produce sour natural gas, i.e. natural gas comprising H2S and optionally other contaminants (e.g. other sulfurous compounds like COS, CS2, RSH, etc.). Natural gas is a general term that is applied to mixtures of light hydrocarbons and optionally other gases (nitrogen, carbon dioxide, helium) derived from natural gas wells. The main component of natural gas is methane. Further, often other hydrocarbons such as ethane, propane, butane or higher hydrocarbons are present.
The Claus process is frequently used for the treatment of hydrogen sulfide recovered from various gas streams, such as hydrocarbon streams, for example natural gas. The multi-step process produces sulfur from gaseous hydrogen sulfide.
The Claus process comprises two steps, a first thermal step and a second catalytic step. In the first thermal step, a portion of the hydrogen-sulfide in the gas is oxidized at temperatures above 850° C. to produce sulfur dioxide and water:2H2S+3O2→2SO2+2H2O  (I).
In the second catalytic step, the sulfur dioxide produced in the thermal step reacts with hydrogen sulfide to produce sulfur and water:2SO2+4H2S→6S+4H2O  (II).
The gaseous elemental sulfur produced in step (II) may be recovered in a condenser, initially as liquid sulfur before further cooling to provide solid elemental sulfur. In some cases, the second catalytic step and sulfur condensing step can be repeated more than once, typically up to three times to improve the recovery of elemental sulfur.
The second catalytic step of the Claus process requires sulfur dioxide, one of the products of reaction (I). However, hydrogen sulfide is also required. Typically, approximately one third of the hydrogen sulfide gas is oxidized to sulfur dioxide in reaction (I), in order to obtain the desired 1:2 molar ratio of sulfur dioxide to hydrogen sulfide for the reaction to produce sulfur in the catalytic step (reaction (II)). The residual off-gases from the Claus process may contain combustible components and sulfur-containing compounds, for instance when there is an excess or deficiency of oxygen (and resultant overproduction or underproduction of sulfur dioxide). Such combustible components can be further processed, suitably in a Claus off-gas treating unit, for instance in a Shell Claus Off-gas Treating (SCOT) unit. The overall reaction for the Claus process can therefore be written as:2H2S+O2→2S+2H2O  (III).